This invention relates to optical fibers, and more particularly to transmission media comprising optical fibers.
Large bandwidth transmission and small size are well-recognized advatages of optical fibers as transmission media. These characteristics make optical fibers a desirable replacement for wire cables especially in congested areas where increased transmission is needed, but where additional space in cable ducts is not available.
One problem confronting the practical implementation of optical fibers, especially where the optical transmitting medium is to be drawn through ducts and thereby subjected to longitudinal and transversal mechanical loads, is that optical fibers are made of very delicate material, i.e. typically fused silica or other glasses. Glass fibers, though desirable for their optical transmitting properties, have less desirable mechanical characteristics as a transmitting medium. While the tensile strength of glass fibers is theoretically very high, their actual tensile strength (typically 2.1 .times. 10.sup.6 g/cm.sup.2 in kilometer lengths) is considerably lower and varies under field conditions. Also glass fibers are subject to static fatigue; that is, in the presence of moisture, glass will fracture under sustained stresses below the instantaneous tensile strength because of growth of surface flaws. Furthermore, glass fibers in very long lengths exhibit a low strain at break, usually less than half of one percent elongation before fracture. These characteristics present serious problems which must be overcome if optical fibers are to be implemented in future optical communication systems. It is likely that many signal channels will be allocated to each fiber in the future which means a fracture in one fiber would mean total communication loss of the channels transmitted in that fiber.
Another aspect is that even when an optical fiber does not fracture under the externally applied stresses, sufficient amplitude in a critical wavelength range may be present in random bends of the fiber axis to result in optical transmission loss. See W. B. Gardner's "Microbending Loss in Optical Fibers," The Bell System Technical Journal, Vol. 54, No. 2, February 1975, pp. 457-465 for a discussion of this phenomenon. This phenomenon can considerably degrade the transmission performance of the fibers, especially over long distances.
Therefore, it is desirable to design an optical communication cable which renders optical fibers a practical transmitting medium. It is especially desirable that the cable be capable of withstanding the tensile forces expected during installation as well as being sufficiently small cross-sectionally to minimize the space occupied in the ducts.
While achieving the foregoing however, it is also necessary to arrange the several fibers in each core in a geometry that facilitates fast, easy, reliable, and low loss splicing of one core to another, or of one portion of one core to a mating portion of another core.
Therefore, one object of the present invention is an optical communication cable which renders optical fibers a reliable yet economically feasible transmitting medium. A second inventive object is to minimize the chances of strain on the fibers under expected loading conditions. A third inventive object is to minimize random bending loss in the fibers. A fourth inventive object is to configure an optical communication cable in such a way as to facilitate mass splicing.